1. Field of the Invention
The present invention relates to the field of medical instrumentation and more specifically to portable apparatus for monitoring cardiac signals for transmission to remote locations.
More particularly, this invention describes a battery operated cardiac signal transmitting apparatus adapted for use by the pacemaker patient which detects both electrocardigram pulses (hereinafter EKG pulses or signals) and electrical stimulus pulses produced by the pacemaker (hereinafter artifact pulses or signals), and efficiently converts them into corresponding acoustic signals for transmission to remote locations such as a physician's office or clinic for analysis.
2. Description of the Prior Art
The use of cardiac pacers to sustain life in heart disease cases is well known. It is also well known that cardiac pacers provide an electrical stimulation to the heart and that when the heart beats normally, or in response to an electrical stimulation pulse, it provides an electrical waveform called an EKG pulse. In the prior art, various means of sensing and recording both the pacer artifact pulses and the EKG pulses have been available. It has also been found advantageous to provide equipment which makes the information concerning this sensed artifact and EKG pulses available to the patient, and to transmit such information to a remote station such as a doctor's office, for example by a telephone system. One example of a telephone transmission system is U.S. Pat. Application Ser. No. 235,252, filed Mar. 16, 1972, abandoned for Application Ser. No. 401,648, filed Sept. 28, 1973 and now U.S. Pat. No. 920,005, entitled "EVALUATION SYSTEM FOR CARDIAC STIMULATORS", and assigned to the Assignee of this invention.
U.S. Pat. No. 3,923,041 to Stasz, et al, assigned to the same Assignee as the instant application, also discloses apparatus for detecting cardiac signals and for transmission of the signals to a remote location. The referenced patent further teaches the use of circuitry for providing a quantitative measure of the artifact pulse repetition rate. More precisely, Stasz, et al discloses a dual channel cardiac signal transmitter producing an acoustic output wherein each artifact pulse received and processed is applied to comparator circuitry configured to generate a time gate, which occurs at a predetermined delayed time, within which each subsequent artifact pulse must occur. Thus, any change in the repetition rate of the artifact pulses, due for example to partial depletion of the implanted battery, is detected.
U.S. Pat. No. 3,885,522 to Kennedy, discloses a system for monitoring various cardiac signal parameters and teaches the organization of an apparatus wherein substantially all of the complex signal processing and analysis is accomplished in the unit which is directly coupled to the patient.
While the monitoring and analysis devices available to the pacemaker patient, and/or the patient with heart impairment of some kind, present valuable techniques for providing close follow up monitoring of the patient by the physician, most are sufficiently complex as to make them unavailable as highly portable and fully patient operable moitoring means.
In the prior art, monitoring and analysis devices are available that are powered by self-contained sources such as a battery and include monitoring circuits for detecting the voltage level output of the battery. Such devices detect the patient's cardiac or EKG signals, as well as the artifact pulses as generated by the patient's artificial heart pacemaker. These detected signals are amplified to drive an acoustical transducer in the form of a speaker, which is coupled to the receiver of a normal telephone, whereby such signals are conveyed via common telephone lines to the doctor's office. A problem with such known units is the relatively low drive capability in terms of power applied to its speaker. In the prior art, the signals to be transmitted are applied to a voltage controlled oscillator (VCO), the frequency of whose output varies as a function of the amplitude of the detected input signals. The VCO output is applied via a pair of amplifying transistors directly across the coils of the speaker to ground. Such an arrangement limits the power that may be applied to the speaker; in particular, the power output of the VCO is relatively limited thereby reducing the power to be applied to the speaker. As a result, the quality of the signal transmitted via the telephone lines suffers in terms of its signal to noise ratio. This problem is further complicated by the fact that the sensing device is energized by a battery, whose voltage level decreases with use and time. Thus, as the battery is progressively used, the voltage level and thus the power applied to the speaker becomes attentuated with the result of the quality of the transmitted signal decreases. Thus, the signal received and processed at the doctor's office will eventually become of such quality as to be unintelligible. Though such devices include monitoring circuits sensing the battery voltage to defeat the VCO and further, the VCO circuits are designed to apply a regulated output, there still exists the basic problem of supplying sufficient power to drive the output speaker.